This invention relates to an electrode formed of a sintered mass of a metal oxide to be used for coating by cationic electrodeposition and to a method for electrodeposition coating by use of the electrode mentioned above.
In recent years, increasing importance has come to be attached to the improvement in anti-corrosion in the case of coating automobile bodies. The measures for anti-corrosion have been studied in terms of base materials, chemical treatments, types of paints, manners of paint application, automobile body designs, etc. from various angles. Among others, the technique of electrodeposition coating constitutes the most effective and economic method for rendering anti-corrosive the inner surfaces of complicated and hollow structures such as automobile bodies, for example, and even those portions which do not readily permit spray coating. Thus, the electrodeposition coating is extensively used today.
The conventional method for electrodeposition coating has preponderantly used anionic electrodepositing paints in consideration of the low cost of paints so used, the relatively low temperature for baking paints, and the relatively low cost of equipment involved. In accordance with the method for anionic electrodeposition coating, however, the article subjected to coating which is used as an anode is dissolved out in the course of electrodeposition coating, whereas the cathode such as of iron immersed in the electrodepositing cell or paint is not dissolved out. Consequently, the effect of the chemically formed coat is degraded and the thickness of the coat formed on the surface of the article to be coated under treatment is small. Accordingly, with the progressive aggravation of the corrosive environment, it has been proved that the conventional anionic electrodeposition coating is not necessarily satisfactory. For this reason, the technique of cationic electrodeposition coating has recently come to find increasing acceptance.
To effect the cationic electrodeposition coating, a water-insoluble polyamine resin, R--NH.sub.2, is obtained by adding a primary amine or secondary amine to the glycidyl group of a water-insoluble resin such as, for example, a bisphenol type epoxy resin thereby effecting ring cleavage thereof, and then an organic acid such as acetic acid or lactic acid is caused to react, as a neutralizing agent (water-solubilizing agent) AH, with the aforementioned water-insoluble polyamine resin to produce an aqueous resin, R--NH.sub.3.sup.+, as shown by the following reaction formula (I). EQU R--NH.sub.2 +AH.fwdarw.R--NH.sub.3.sup.+ +A.sup.- (I)
In a cationic electrodepositing paint solution formed of the aforementioned water-soluble resin, and if necessary, a crosslinking agent and a pigment, an article to be coated is immersed as a negatively charged electrode (hereinafter referred to as "cathode"). Separately a positively charged electrode (hereinafter referred to as "anode") such as of stainless steel or carbon is immersed in the same solution. Electric current is passed between the cathode (the article under treatment) and the anode.
By the passage of the electric current, the positively charged paint components electrophoretically migrate in the solution and, on arrival at the article (the cathode) coagulates and precipitates by emitting the electric charges as shown by the following formula (II) and gives rise to a water-insoluble coat on the article. EQU 2H.sub.2 O+2e.sup.- .fwdarw.H.sub.2 +2OH.sup.- EQU OH.sup.- +R--NH.sub.3 .fwdarw.R--NH.sub.2 +H.sub.2 O (II)
On the anode which is made of a metal such as, for example, stainless steel as indicated in the formula (III), generation of metal ions and simultaneous evolution of oxygen shown by the formula (IV) are observed. EQU M.fwdarw.M.sup.n+ +ne.sup.- (III) EQU 4OH.sup.- .fwdarw.O.sub.2 .uparw.2H.sub.2 O+4e.sup.- (IV)
In case the anode is made of carbon, since it is not a metal, the dissolution indicated by the formula (III) does not occur, but the evolution of oxygen through the reaction of the formula (IV) does occur. Consequently, the carbon of the anode itself is oxidized. Therefore, with the lapse of time, the anode loses its weight and eventually a flaw is developed. Particularly in the case of an anode made of a metal, the metal ions dissolved out from the anode get mixed into the solution. When the paint component is coagulated and precipitated, these metal ions are simultaneously coagulated and precipitated to the article. The coat which is consequently obtained suffers from poor anti-corrosion property or coarse coating surface. In the case of an anode made of carbon, the oxidation causes the anode to shed fine carbon particles into the solution. If the electrodeposition coating is continued with carbon particles contained in the solution, gritty prominences stand out on the surface of the coated article, with the rsult that the produced coat suffers from inferior appearance and deficient anti-corrosion property.
As materials for the anode which avoid release of metal ions, the use of high-grade stainless steel of SUS-316 or the like, or a noble metal such as platinum may be considered. Stainless steel, in addition to being expensive, is inevitably susceptible to release of metal ions, if only to a slight extent. The noble metal is too expensive to be feasible for the use contemplated. Carbon and graphite have a problem that they have poor processibility.